The root item can be anything that inherits QGraphicsItem. For example, in this case it is QGraphicsWidget which is a plasmoid. It can also simply be an Item. I also noticed that PathView does not respond to mouse inputs automatically (so flicking doesn't work). Probably because events are being intercepted. So take note, it'll have to be e.g. an Item, for that case.

See the KDE Examples git repository for more KDE-related helpful resources.

Layouts

Row and Column

Anchors

Anchor layouts offer a nice way of grouping UI elements nicely together. The idea is that you connect edges or corners of one element to the edge or corner of another widget.
Some examples:

qmlviewer

Where the -I is the path to the plasma plugin for qml. Try to look for the path of
/usr/lib/kde4/imports/org/kde/plasma/graphicswidgets/libgraphicswidgetsbindingsplugin.so
and use everything up to org of that path.

Hovewer it's strongly discouraged to use qmlviewer to develop plasmoids, because some features won't be available there:

localization with i18n()

access to the global plasmoid object

device specific qml files imported with plasmapackage:// urls

bindings for qicons, KJobs, services and KConfig

Features only available in Plasma widgets

In order to have a better integration with the KDE platform and to reach an higher degree of expressivity, the stock features of QML have been expanded with the following features, that strictly follow the Plasmoid JavaScript API:

Plasmoid object

Every QML plasmoid will have an object called plasmoid, that will give access to the configuration, the formfactor, immutability and so on. It offers the same api as the object with the same name in the Javascript API.

Localization

Extra types

KConfigGroup: it's an object with its cnfig keys readable and writable as properties

QIcon: can be constructed with QIcon("fdo name") such as QIcon("konqueror")

KJob

Plasma Service api

Plasma specific imports

To use some Plasma specific features is necessary to use some particular QML imports.

Plasma Core

org.kde.plasma.core
This is the import that lets you access to the most important Plasma Core features.

DataSource

Used to connect to a dataengine

DataModel

Attaches to a DataSource, makes possible to use a dataengine as a model for a QML ListView, GridView, PathView and so on

Svg

Loads a Plasma Svg, it's not the visual item

SvgItem

Visual item that paints a Svg

FrameSvg

Loads a Plasma FrameSvg, it's not the visual item.

FrameSvgItem

Visual item that displays a Plasma FrameSvg

Extra Qt features

org.kde.qtextraimports

QPixmapItem

QImageItem

QIconItem

Plasma Widgets in QML

To use standard plasma widgets (e.g. Plasma::LineEdit, etc.), you simply add an import line for them.
All properties, signals and slots from ordinary Plasma widgets are available there.
Those widgets are provided as a transition tool, intended to be replaced by the Plasma version of QtComponents.